Austenitic stainless steel having improved strength

ABSTRACT

An austenitic stainless steel with improved strength is disclosed. The austenitic stainless steel includes, in percent (%) by weight of the entire composition, C: 0.02 to 0.14%, Si: 0.2 to 0.6%, S: less than 0.01%, Mn: 2.0 to 4.5%, Ni: 2.5 to 5.0%, Cr: 19.0 to 22.0%, Cu: 1.0 to 3.0%, Mo: less than 1.0%, N: 0.25 to 0.40%, the remainder of iron (Fe) and other inevitable impurities, and the Solubility of Nitrogen in Liquid (SNL) value represented by the following equation (1) is equal to or greater than the content of N. Equation (1): SNL=−0.188−0.0423×C−0.0517×Si+0.012×Mn+0.0048×Ni+0.0252×Cr−0.00906×Cu+0.00021×Mo. Here, C, Si, Mn, Ni, Cr, Cu, and Mo mean the content (% by weight) of each element.

TECHNICAL FIELD

The present disclosure relates to austenitic stainless steel, inparticular, to austenitic stainless steel with improved strength whilesecuring elongation and corrosion resistance.

BACKGROUND ART

Stainless steel refers to steel that has strong corrosion resistance bysuppressing corrosion, a weak point of carbon steel. In general,stainless steel is classified according to its chemical composition ormetal structure. According to the metal structure, stainless steel canbe classified into austenite, ferrite, martensite and dual phase.

Among them, austenitic stainless steel is a steel containing a largeamount of chromium (Cr) and nickel (Ni), and is most commonly used. Forexample, 316L stainless steel has a component based on 16-18% Cr, 10-14%Ni, and 2-3% molybdenum (Mo), is applied in various industrial fields bysecuring corrosion resistance and molding properties.

However, in the case of Ni and Mo, there is a problem in terms of pricecompetitiveness due to high material prices, and raw material supply anddemand are unstable due to extreme fluctuations in material prices, andit is difficult to secure supply price stability.

Therefore, research has been conducted to reduce the content of Ni andMo while securing corrosion resistance and formability of theconventional 316L stainless steel level. As a substitute for such 316Lstainless steel, 200 series stainless steel, for example, 216 steel,which reduced Ni and increased the content of Mn was developed.

216 stainless steel is basically a steel containing a large amount of Mnof 7% or more in order to reduce the material price by reducing the Nicontent to a certain amount or less, and to secure the stability of theaustenite phase according to the amount of Ni reduction. It contains17.5-22% Cr, 5-7% Ni, 7.5-9% Mn and 2-3% Mo in percent (%) by weight.

By this component-based design, 216 stainless steel can secure a levelof corrosion resistance similar to that of 316L stainless steel, but dueto the generation of a large amount of Mn fume during the steel makingprocess due to the addition of a large amount of Mn, not onlyenvironmental improvement is required, but also the production of steelmaking inclusions (MnS) results in a decrease in productivity in themanufacturing process and a decrease in the surface quality of the finalmaterial.

Meanwhile, the duplex stainless steel is a substitute for 316L stainlesssteel.

Duplex stainless steel is a stainless steel having a microstructure inwhich austenite phase and ferrite phase are mixed. Specifically, theaustenite phase and the ferrite phase each exist in a volume fraction ofabout 35 to 65%, showing the characteristics of both austeniticstainless steel and ferritic stainless steel.

Duplex stainless steel secures corrosion resistance equivalent to 316Lstainless steel, and has low Ni content, making it economical and easyto secure high strength. Therefore, it is in the spotlight as a steelfor industrial facilities such as desalination facilities, pulp, paper,and chemical facilities that require corrosion resistance.

In particular, among duplex stainless steels, research on Lean Duplexstainless steel limited to 19-23% Cr, 1.8-3.5% Ni, 0-2% Mn and 0.5-1.0%Mo by reducing expensive alloying elements such as Ni and Mo and, whichfurther highlights the advantages of low alloy cost through the additionof 0.16-0.3% high nitrogen, is being actively conducted.

However, in the case of lean duplex stainless steel, there is a problemin that the formability and elongation are inferior due to the formationof an phase interface between austenite and ferrite. Therefore, it isrequired to develop austenitic stainless steel with improved strengthwhile securing elongation and corrosion resistance while reducing Ni andMo.

DISCLOSURE Technical Problem

Embodiments of the present disclosure are intended to provide anaustenitic stainless steel with improved strength while securingelongation and corrosion resistance of the existing 316L stainless steellevel.

Technical Solution

In accordance with an aspect of the present disclosure, an austeniticstainless steel with improved strength includes, in percent (%) byweight of the entire composition, C: 0.02 to 0.14%, Si: 0.2 to 0.6%, S:less than 0.01%, Mn: 2.0 to 4.5%, Ni: 2.5 to 5.0%, Cr: 19.0 to 22.0%,Cu: 1.0 to 3.0%, Mo: less than 1.0%, N: 0.25 to 0.40%, the remainder ofiron (Fe) and other inevitable impurities, and the Solubility ofNitrogen in Liquid (SNL) value represented by the following equation (1)is equal to or greater than the content of N.

SNL=−0.188−0.0423×C−0.0517×Si+0.012×Mn+0.0048×Ni+

0.0252×Cr−0.00906×Cu+0.00021×Mo  Equation (1)

(Here, C, Si, Mn, Ni, Cr, Cu, and Mo mean the content (% by weight) ofeach element.)

The C+N is 0.5% or less (excluding 0).

The austenitic stainless steel may further include: one or more of B:0.001 to 0.005% and Ca: 0.001 to 0.003%.

The Md₃₀ value represented by the following equation (2) may satisfy −50or less.

Md₃₀=551−462×(C+N)−9.2×Si−8.1×Mn−13.7×Cr−29×(Ni+

Cu)−8.5×Mo  Equation (2)

(Here, C, N, Si, Mn, Cr, Ni, Cu, and Mo mean the content (% by weight)of each element.)

The austenitic stainless steel may satisfy the following equation (3).

Creq/Nieq≤1.8  Equation (3)

(Here, Creq=Cr+Mo+1.5×Si, Nieq=Ni+0.5×Mn+30×(C+N)+0.5×Cu.)

The Pitting Resistance Equivalent Number (PREN) represented by thefollowing equation (4) may satisfy 22 or more.

PREN=16+3.3Mo+16N−0.5Mn  Equation (4)

(Here, Mo, N, and Mn mean the content (% by weight) of each element.)

The yield strength (0.2 off-set) may be 400 to 450 MPa and the tensilestrength may be 700 to 850 MPa.

The elongation may be 35% or more.

Advantageous Effects

According to an embodiment of the present disclosure, it is possible toprovide austenitic stainless steel with improved strength while securingelongation and corrosion resistance of the existing 316L stainless steellevel.

DESCRIPTION OF DRAWINGS

FIG. 1 is a graph for illustrating a correlation between Thermocalc.calculation result and a regression equation applied value for derivingSolubility of Nitrogen in Liquid (SNL) value of austenitic stainlesssteel according to an embodiment of the present disclosure.

MODES OF THE INVENTION

Hereinafter, the embodiments of the present disclosure will be describedin detail with reference to the accompanying drawings. The followingembodiments are provided to transfer the technical concepts of thepresent disclosure to one of ordinary skill in the art. However, thepresent disclosure is not limited to these embodiments, and may beembodied in another form. In the drawings, parts that are irrelevant tothe descriptions may be not shown in order to clarify the presentdisclosure, and also, for easy understanding, the sizes of componentsare more or less exaggeratedly shown.

Throughout the specification, when a part “includes” a certaincomponent, it means that other components may be further included ratherthan excluding other components unless specifically stated to thecontrary.

Expressions in the singular number include expressions in the pluralunless the context clearly has exceptions.

Hereinafter, embodiments according to the present disclosure will bedescribed in detail with reference to the accompanying drawings.

An austenitic stainless steel according to an aspect of presentdisclosure includes, in percent (%) by weight of the entire composition,C: 0.02 to 0.14%, Si: 0.2 to 0.6%, P: less than 0.1%, S: less than0.01%, Mn: 2.0 to 4.5%, Ni: 2.5 to 5.0%, Cr: 19.0 to 22.0%, Cu: 1.0 To3.0%, Mo: less than 1.0%, N: 0.25 to 0.40%, the remainder of iron (Fe)and other inevitable impurities.

Hereinafter, the reason for limiting the numerical value of the contentof the alloying component in the embodiment of the present disclosurewill be described. Hereinafter, unless otherwise specified, the unit is% by weight.

The content of C is 0.02 to 0.14%.

Carbon (C) is an element effective in stabilizing the austenite phase,but when the content is low, 0.02% or more may be added as additionalaustenite stabilizing elements are required. However, if the content isexcessive, workability may be lowered due to the solid solutionstrengthening effect. In addition, if the content is excessive, it mayadversely affect the ductility, toughness, corrosion resistance, etc. byinducing grain boundary precipitation of Cr carbide due to latent heatafter hot-rolled coiling and the heat-affected zone of the weld, so theupper limit may be limited to 0.14%.

The content of Si is 0.2 to 0.6%.

Silicon (Si) serves as a deoxidizing agent during the steelmakingprocess and is an effective element to improve corrosion resistance andcan be added by 0.2% or more. However, Si is an element that iseffective in stabilizing the ferrite phase, and when excessively added,it promotes the formation of delta ferrite in the casting slab, therebyreducing hot workability. In addition, when excessively added, theductility/toughness of the steel material due to the solid solutionstrengthening effect may be lowered, and thus the upper limit thereofmay be limited to 0.6%.

The content of Mn is 2.0 to 4.5%.

Manganese (Mn) is an austenite phase stabilizing element that is addedinstead of nickel (Ni) in the present disclosure. It is effective inimproving cold rolling properties by suppressing the generation ofstrain-induced martensite, and is an element that increases thesolubility of nitrogen (N) during a steelmaking process to be describedlater, and may be added by 2.0% or more. However, if the content isexcessive, Mn may reduce the ductility, toughness, and corrosionresistance of steel materials as it causes an increase in S-basedinclusions (MnS), and thus the upper limit thereof may be limited to4.5%.

The content of Ni is 2.5 to 5.0%.

Nickel (Ni) is a strong austenite phase stabilizing element and isessential to secure good hot workability and cold workability. Inparticular, even when a certain amount of Mn is added, it is essentialto add 2.5% or more. However, since Ni is an expensive element, itcauses an increase in raw material cost when a large amount is added.Accordingly, the upper limit can be limited to 5.0% in consideration ofboth cost and efficiency of the steel.

The content of Cr is 19 to 22%.

Although chromium (Cr) is a ferrite stabilizing element, it is effectivein suppressing the formation of martensite phase, and is a basic elementthat secures corrosion resistance required for stainless steel. Inaddition, 19% or more may be added as an element that increases thesolubility of nitrogen (N) during a steelmaking process to be describedlater. However, if the content is excessive, the manufacturing costincreases, and the formation of delta (δ) ferrite in the slab leads to adecrease in hot workability. Accordingly, there is a problem thatadditional addition of austenite stabilizing elements such as Ni and Mnis required, and the upper limit thereof can be limited to 22%.

The content of P is less than 0.1%.

As phosphorus (P) lowers corrosion resistance or hot workability, itsupper limit may be limited to 0.1%.

The content of S is less than 0.01%.

As sulfur (S) lowers corrosion resistance or hot workability, its upperlimit may be limited to 0.01%.

The content of Cu is 1.0 to 3.0%.

Copper (Cu) is an austenite phase stabilizing element added instead ofnickel (Ni) in the present disclosure, and improves formability byimproving corrosion resistance in a reducing environment and reducingStacking Fault Energy (SFE). 1.0% or more may be added to sufficientlyexpress such an effect. However, if the content is excessive, the upperlimit may be limited to 3.0% because it may increase the material costas well as lower the hot workability.

The content of Mo is less than 1.0%.

Molybdenum (Mo) is an effective element in improving the corrosionresistance of stainless steel by modifying the passive film. However,since Mo is an expensive element, when a large amount of Mo is added, itcauses an increase in raw material cost and has a problem ofdeteriorating hot workability. Accordingly, in consideration of thecost-efficiency and hot workability of the steel, the upper limit can belimited to 1.0%.

The content of N is 0.25 to 0.40%.

Nitrogen (N) is an element that is effective in improving corrosionresistance and is a strong austenite stabilizing element. Therefore,nitrogen alloying can reduce material cost by enabling lower use of Ni,Cu, and Mn. 0.25% or more may be added to sufficiently express thiseffect. However, if the content is excessive, since workability andmoldability may be deteriorated due to the solid solution strengtheningeffect, the upper limit may be limited to 0.40%.

The content of C+N is 0.5% or less.

C and N are elements that are effective for improving strength, but whenthe content is excessive, there is a problem of lowering theworkability, and the upper limit of the total may be limited to 0.5%.

In addition, the austenitic stainless steel with improved strengthaccording to an embodiment of the present disclosure may further includeone or more of B: 0.001 to 0.005 and Ca: 0.001 to 0.003%.

The content of B is 0.001 to 0.005%.

Boron (B) is an element effective in securing good surface quality bysuppressing the occurrence of cracks during casting, and can be added by0.001% or more. However, if the content is excessive, nitride (BN) maybe formed on the product surface during the annealing/pickling process,thereby reducing the surface quality. Therefore, the upper limit can belimited to 0.005%.

The content of Ca is 0.001 to 0.003%.

Calcium (Ca) is an element that improves product cleanliness bysuppressing the formation of MnS steel-making inclusions generated atgrain boundaries when high Mn is contained, and can be added by 0.001%or more. However, if the content is excessive, it may cause a decreasein hot workability and a decrease in product surface quality due toformation of Ca-based inclusions, and the upper limit may be limited to0.003%.

The remaining component of the present disclosure is iron (Fe). However,since unintended impurities from the raw material or the surroundingenvironment may inevitably be mixed in the normal manufacturing process,this cannot be excluded. Since these impurities are known to anyone ofordinary skill in the manufacturing process, all the contents are notspecifically mentioned in the present specification.

In order to secure price competitiveness of austenite stainless steel,it is necessary to reduce the content of expensive austenite stabilizingelements such as Ni and Mn, and it is required to predict the amount ofN added that can compensate for this. To this end, it is necessary toset the optimal N content through calculation of the solubility limit ofN in consideration of each alloy component.

Thus, using the state diagram prediction program Thermocalc., thecontent of N that can be dissolved in the molten metal temperature at1150° C. is derived according to the amount of each alloy element (C,Si, Mn, Ni, Cr, Cu, Mo) added.

FIG. 1 is a graph for illustrating a correlation between Thermocalc.calculation result and a regression equation applied value for derivingSolubility of Nitrogen in Liquid (SNL) value of austenitic stainlesssteel according to an embodiment of the present disclosure.

Referring to FIG. 1, the limit value at which nitrogen is dissolved inthe molten metal is calculated and expressed as “N solubility limit(The.)”.

The SNL (Solubility of Nitrogen in Liquid) regression equation ofEquation (1) was derived based on the calculated value of Thermocalc.according to the component change.

SNL=−0.188−0.0423×C−0.0517×Si+0.012×Mn+0.0048×Ni+

0.0252×Cr−0.00906×Cu+0.00021×Mo  Equation (1)

When applying the derived regression equation, it was confirmed that theR(sq) value corresponds to a high correlation of 100%. In addition, itwas confirmed that it is possible to secure suitability in therelationship between the calculation result of the thermocalc for eachcomponent to derive SNL, which is N melting limit value, and theregression equation.

In the austenitic stainless steel with improved strength according to anembodiment of the present disclosure, the SNL value is greater than orequal to N content. In this way, when the SNL value was set higher thanthe N content to increase the nitrogen solubility limit, it wasconfirmed that the steelmaking operation of the target alloy componentwas performed satisfactorily.

In the case of austenitic stainless steel, it is applied to productsthat require a beautiful surface. For products that require a beautifulsurface, it is common to perform a bright annealing on cold-rolledmaterials. This bright annealing is a heat treatment technology thatkeeps the surface bright and beautiful without changing the color andproperties of the surface by preventing reoxidation occurring during theheat treatment process of the stainless steel cold rolled material byperforming heat treatment on the stainless steel cold rolled material ina reducing atmosphere (Dew point −40˜−60° C.) using nitrogen (N₂),hydrogen (H₂), etc. Bright annealing using hydrogen as the atmospheregas used for bright annealing is the most common, because it is mostwidely used for suppressing discoloration of the surface as well as highheat capacity.

Compared to general austenitic stainless steel, in stainless steel thathas reduced austenite stabilizing elements such as Ni and Mn as in thepresent disclosure, there is a point to be considered when applyingbright annealing in a hydrogen atmosphere.

During bright annealing, there is a high possibility of inferiorworkability due to hydrogen embrittlement defects in the final materialdue to the penetration of hydrogen. In the case of stainless steel withreduced austenite stabilizing elements such as Ni and Mn, during coldrolling before final bright annealing, stress-induced martensite orstrain-induced martensite is formed around the surface layer. Themartensite phase formed on the surface layer is in contact with hydrogenatoms, which are inert gases, before being transformed into an austenitephase by heat treatment during bright annealing. Some of these hydrogenatoms penetrate into the martensite phase. As the existingstress-induced martensite or strain-induced martensite isphase-transformed into the austenite phase by bright annealing, hydrogenatoms that have penetrated inside cannot be discharged to the outsideand are trapped in the atomic state at the surface.

The hydrogen atoms penetrating into the surface layer are naturallybake-out after a certain period of time at room temperature for ferriteor martensite phase, which are general BCC and BCT structures, and donot significantly affect the physical properties.

On the other hand, when the martensite phase of the surface layer istransformed into an austenite phase by bright annealing, that is, whenhydrogen atoms are present in the lattice structure of FCC, even after aconsiderable amount of time has passed at room temperature, the naturalbakeout of hydrogen atoms is not smooth and remains in the material fora long time.

This hydrogen atom is known as a factor causing hydrogen embrittlement.Hydrogen atoms trapped in the material due to some processing ordeformation change to the state of hydrogen molecules (gas), and when acertain pressure is reached, it acts as a starting point of cracks undera certain load, causing a decrease in elongation.

Therefore, for austenitic stainless steel with relatively low Ni and Mn,the beautiful surface quality and workability can be secured throughbright annealing only by controlling the amount of martensite phaseformed on the surface by work hardening together with the alloycomponent.

Accordingly, for the austenitic stainless steel with improved strengthaccording to an embodiment of the present disclosure, the Md30 valueexpressed by the following equation (2) satisfies the range of −50° C.or less.

Md₃₀=551−462×(C+N)−9.2×Si−8.1×Mn−13.7×Cr−29×(Ni+Cu)−

8.5×Mo  Equation (2)

In austenitic stainless steel, martensitic transformation occurs byplastic working at a temperature of the martensitic transformationinitiation temperature (Ms) or more. The upper limit temperature thatcauses phase transformation by such processing is represented by the Mdvalue, and is a criterion of the degree to which phase transformationoccurs by processing.

In particular, the temperature (° C.) at which 50% phase transformationto martensite occurs when 30% strain is applied is defined as Md₃₀. Whenthe Md₃₀ value is high, it is easy to form the strain-induced martensitephase, whereas when the Md₃₀ value is low, the strain-induced martensitephase is relatively difficult to form. In general, the Md30 value isused as an index to determine the austenite stability of austeniticstainless steel, and can be calculated through the Nohara regressionequation expressed by the equation (2).

The reason why various kinds of phases are formed by the difference inalloy component content is because the effect of each added alloycomponent on the phase balance is different.

The degree to which each alloy component affects the phase balance canbe calculated through Creq and Nieq, and the phase generated at roomtemperature can be predicted through the Creq/Nieq ratio expressed as inthe equation (3) below.

Creq/Nieq≤1.8  Equation (3)

Here, Creq=Cr+Mo+1.5×Si, Nieq=Ni+0.5×Mn+30×(C+N)+0.5×Cu.

That is, when the Creq/Nieq ratio is low, austenite single phase can beformed at room temperature due to relatively high austenite stability.When the Creq/Nieq ratio is high, the austenite stability is low and theferrite phase is likely to be formed locally.

As a result of reviewing by applying the Creq/Nieq ratio to variousalloy components, present inventor confirmed that the formation ofaustenite single-phase matrix structure was possible when the Creq/Nieqratio was 1.8 or less.

Various methods are used as a criterion for evaluating the corrosionresistance of stainless steel, but the use of the Pitting ResistanceEquivalent Number (PREN) is a simple method of examining thediscrimination power of alloy components.

PREN is generally used to influence Cr, Mo, and N, but for steel gradeswith relatively high Mn content, since it is necessary to consider theinfluence of Mn as well, the following equation (4) was derived from thepresent disclosure.

When the generally used high corrosion resistance 316L stainless steelalloy composition is applied to the following equation, it shows a valueof about 22. Therefore, in the present disclosure, the PREN value wasset to 22 or higher in order to secure corrosion resistance equal to orhigher than that of 316L stainless steel.

PREN=16+3.3Mo+16N−0.5Mn  Equation (4)

Hereinafter, the present disclosure will be described in more detailthrough examples.

For various alloy component ranges shown in Table 1 below, a slab havinga thickness of 200 mm was prepared by melting an ingot, heated at 1,240°C. for 2 hours, and then hot-rolled to prepare a hot-rolled steel sheethaving a thickness of 3 mm.

TABLE 1 C Si Mn S Ni Cr Cu Mo N C + N inventive 0.104 0.48 2.91 0.0053.53 20.8 2.1 0.52 0.3 0.404 example 1 inventive 0.103 0.49 3.4 0.0053.35 19.6 1.16 0.39 0.27 0.373 example 2 inventive 0.088 0.31 3.41 0.0043.7 21.7 2.51 0.10 0.34 0.428 example 3 inventive 0.035 0.31 3.8 0.0064.2 21 2.48 0.20 0.33 0.365 example 4 comparative 0.02 0.52 1.4 0.00410.4 16.6 0.39 2.00 0.018 0.038 example 1 comparative 0.014 0.55 2.40.006 2.4 20.3 0.1 1.30 0.2 0.166 example 2 comparative 0.1 0.38 3.80.006 3.4 17.2 1.45 0.10 0.21 0.310 example 3 comparative 0.15 0.46 3.80.004 3.6 21.6 2.04 0.32 0.35 0.500 example 4

After performing a solution treatment at 1,150° C. for 1 minute,microstructure observation and evaluation of various mechanicalproperties were performed.

Mechanical properties were measured using a No. 5 test piece specifiedin Japanese Industrial Standard JIS Z 2201. Specifically, a tensile testwas conducted using JIS Z 2201, and the measured yield strength, tensilestrength, and elongation were described in Table 2 below.

In addition, SNL calculation results, Md₃₀ calculation results,Creq/Nieq ratio calculation results, and PREN calculation results for 4inventive examples and 4 comparative examples in Table 1 are shown inTable 2 below.

TABLE 2 N N solubility solubility Mechanical properties Steel limitlimit Md₃₀ Creq/ Phase YS TS EI grade (The.) (Reg.) (° C.) Nieq PRENanalysis (MPa) (MPa) (%) inventive 0.3238 0.3244 −121 1.2140 25.861Austenite 490 780 44% example 1 inventive 0.3067 0.3080 −60 1.232223.507 Austenite 460 760 50% example 2 inventive 0.3582 0.3590 −1701.0914 25.765 Austenite 510 800 44% example 3 inventive 0.3472 0.3488−136 1.1845 25.04 Austenite 470 750 42% example 4 comparative 0.22050.2204 −60 1.5585 22.788 Austenite 220 540 58% example 1 comparative0.3230 0.3233 76 2.6076 25.822 Duplex 480 700 45% example 2 comparative0.2552 0.2556 −5 1.1661 18.99 Austenite 380 720 54% example 3comparative 0.3544 0.3550 −180 1.0507 26.356 Austenite 530 830 32%example 4

In the case of comparative example 1, which corresponds to thecomposition of general 316L stainless steel, it represents the tissuecomposed of the austenite phase, and it can be seen that the PREN valueis 22 or higher. However, less than 0.25% of nitrogen was added, and themechanical property evaluation result showed a yield strength of 220 MPaand a tensile strength of 540 MPa. This corresponds to the physicalproperties of generally widely used soft austenitic stainless steel, andthus has a problem that is difficult to apply to materials requiringhigh strength.

In the case of comparative example 2 in which the Creq/Nieq ratioexceeds 1.8, as Mo is added above a certain level, the PREN value isabout 26, indicating excellent pitting resistance. In addition, it canbe seen that the mechanical property evaluation results showed a yieldstrength of 480 MPa, a tensile strength of 700 MPa, and an elongation of45%.

However, as an alloy component in which both Ni and N are relativelylow, when observing the microstructure at room temperature, it wasconfirmed that the austenite phase and the ferrite phase formed a duplexstructure with about 5:5. This is because the stabilization of ferritein the phase balance is relatively higher than that of 316L stainlesssteel. In the duplex structure, cracks may occur at the interfacebetween the austenite phase and the ferrite phase, so there is a problemthat it is difficult to apply to materials requiring molding such asbending.

In the case of comparative example 3, in which the content of Ni and Mnwas slightly increased compared to comparative example 2 and theCreq/Nieq ratio was set to 1.8 or less, when the microstructure wasobserved, a structure composed of austenite phase was formed, and themechanical properties were harder than 316L of comparative example 1,and softer than the duplex stainless steel of comparative example 2.

However, the Md₃₀ value is −5° C., and hydrogen embrittlement is likelyto occur when producing bright annealing materials with beautifulsurfaces in the future. In addition, since the N solubility limit, whichis greatly affected by the Cr content, is low, the amount of N added is0.21%, and the nitrogen factor of the PREN value cannot be maximized,making it difficult to secure pitting resistance of 316L level.

In the case of comparative example 4, in which the contents of N, C, andCr were increased compared to comparative example 3, it is suitable formanufacturing bright annealing materials as it shows the Md₃₀ value atthe level of −180° C., and by setting the Creq/Nieq ratio to 1.8 orless, it can be seen that austenite single-phase structure can besecured.

However, it can be seen that the C+N content is 0.5%, exceeding 0.5%,which is the upper limit of the present disclosure, indicating hardmechanical properties and elongation of less than 35%.

Referring to Table 2, in the case of inventive examples 1 to 4 of thepresent disclosure, it is possible to secure Md₃₀ value below −50° C.,so the possibility of hydrogen embrittlement is low during brightannealing. In addition, the ratio of the nickel equivalent (Nieq) andthe chromium equivalent (Creq) (Creq/Nieq) satisfies the range of 1.8 orless, so that the austenite single-phase structure can be formed at roomtemperature.

In addition, it was confirmed that the content of Ni and Mo isrelatively low, and while securing price competitiveness, it has a PRENvalue of 22 or more. As a result of mechanical property evaluation, itwas confirmed that it was possible to realize high-strengthcharacteristics compared to 316L and secure good elongation of 35% ormore.

From the above results, for austenitic stainless steel including, inpercent (%) by weight of the entire composition, C: 0.02 to 0.14%, Si:0.2 to 0.6%, P: less than 0.1%, S: less than 0.01%, Mn: 2.0 to 4.5%, Ni:2.5 to 5.0%, Cr: 19.0 to 22.0%, Cu: 1.0 to 3.0%, Mo: less than 1.0%, N:0.25 to 0.40%, the remainder of iron (Fe) and other inevitableimpurities, it can secure the processability and corrosion resistance ofthe existing 316L stainless steel level through SNL value control forsecuring price competitiveness and ease of steel making newly proposedby the present disclosure, Md30 value control for securing austenitephase stability, Creq/Nieq ratio control for forming austenite phase inmicrostructure, and PREN control for securing corrosion resistance. Inaddition, it can be seen that stainless steel that can improve pricecompetitiveness and strength can be manufactured.

In the foregoing, exemplary inventive examples of the present disclosurehave been described, but the present disclosure is not limited thereto,and a person with ordinary knowledge in the relevant technical fielddoes not depart from the concept and scope of the following claims. Itwill be appreciated that various changes and modifications are possiblein.

1. An austenitic stainless steel with improved strength comprising, inpercent (%) by weight of the entire composition, C: 0.02 to 0.14%, Si:0.2 to 0.6%, S: less than 0.01%, Mn: 2.0 to 4.5%, Ni: 2.5 to 5.0%, Cr:19.0 to 22.0%, Cu: 1.0 to 3.0%, Mo: less than 1.0%, N: 0.25 to 0.40%,the remainder of iron (Fe) and other inevitable impurities, and whereinthe Solubility of Nitrogen in Liquid (SNL) value represented by thefollowing equation (1) is equal to or greater than the content of N,SNL=−0.188−0.0423×C−0.0517×Si+0.012×Mn+0.0048×Ni+0.0252×Cr−0.00906×Cu+0.00021×Mo  Equation (1) (Here, C, Si, Mn, Ni, Cr,Cu, and Mo mean the content (% by weight) of each element.)
 2. Theaustenitic stainless steel of claim 1, wherein the C+N is 0.5% or less(excluding 0).
 3. The austenitic stainless steel of claim 1, furthercomprising: one or more of B: 0.001 to 0.005% and Ca: 0.001 to 0.003%.4. The austenitic stainless steel of claim 1, wherein the Md₃₀ valuerepresented by the following equation (2) satisfies −50 or less,Md₃₀=551−462×(C+N)−9.2×Si−8.1×Mn−13.7×Cr−29×(Ni+Cu)−8.5×Mo  Equation (2) (Here, C, N, Si, Mn, Cr, Ni, Cu, and Mo meanthe content (% by weight) of each element.)
 5. The austenitic stainlesssteel of claim 1, wherein the austenitic stainless steel satisfies thefollowing equation (3),Creq/Nieq≤1.8  Equation (3) (Here, Creq=Cr+Mo+1.5×Si,Nieq=Ni+0.5×Mn+30×(C+N)+0.5×Cu.)
 6. The austenitic stainless steel ofclaim 1, wherein the Pitting Resistance Equivalent Number (PREN)represented by the following equation (4) satisfies 22 or more,PREN=16+3.3Mo+16N−0.5Mn  Equation (4) (Here, Mo, N, and Mn mean thecontent (% by weight) of each element.)
 7. The austenitic stainlesssteel of claim 1, wherein the yield strength (0.2 off-set) is 400 to 450MPa and the tensile strength is 700 to 850 MPa.
 8. The austeniticstainless steel of claim 1, wherein the elongation is 35% or more.